Major affective disorders are common, severe, chronic and often life-threatening illnesses. Major depression contributes to significant morbidity and mortality. Impairment in physical and social functioning resulting from depression can be just as severe as other chronic medical illnesses. Suicide is the cause of death in 10-20% of individuals with either bipolar or recurrent depressive disorders. Despite the availability of a wide range of antidepressant drugs, clinical trials indicate that 30% to 40% of patients with major depression fail to respond to first-line antidepressant treatment, despite adequate dosage, duration, and compliance. Thus, there is a clear need to develop novel and improved therapeutics for unipolar and bipolar depression. Recent preclinical studies suggest that antidepressants may exert delayed indirect effects on the glutamatergic system. Furthermore, a growing body of data suggests that mood disorders are associated with regional volumetric reductions, and cell loss and atrophy. It is thus noteworthy that lamotrigine, which, among other effects reduces glutamate release, has antidepressant effects, and a pilot study has suggested that N-methyl-D-aspartatic acid (NMDA) antagonists may have antidepressant effects. Together, this data suggests that the glutamatergic system may play a role in the pathophysiology and treatment of depression, and that agents which more directly reduce glutamatergic neurotransmission may represent a novel class of antidepressants. In an open-label study, we tested riluzole, an agent that is Food and Drug Administration-approved for Amyotrophic Lateral Sclerosis that has significant antiglutamatergic and neuroprotective properties in patients with treatment-resistant major depression and found that it had significant antidepressant properties. Our research now extends to test NMDA antagonists in major depression in a placebo-controlled trial. We are obtaining neuroimaging, neuropsychological, neurophysiological, and genetic data. Nineteen treatment-resistant depressed patients were recruited for this protocol (53% of them were classified as Stage 2 treatment-resistance). They received riluzole at a mean daily dose of 169 mg. Significant improvement occurred on weeks 3 through 6 for all patients and weeks 2 through 6 for completers (all p<0.05). These preliminary results indicate that riluzole may have antidepressant properties in some patients. Our research now extends to test NMDA antagonists in major depression in a placebo-controlled trial. We are obtaining neuroimaging, neuropsychological, neurophysiologic, and genetic data. Similarly, we are investigating the efficacy of Riluzole in a population of Bipolar Disorder patients.